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    Join Date
    Aug 2008
    East of Branson, Missouri
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    Default Informational - Relays, fuses, switches, aux lighting, fog lights, etc!

    Well, it comes up once in a while so I figured we'd get this down to making a thread about it to refer people to.

    Questions like "How do I hook up fog lights?", "How do I power this", "How do I have these turn on when this is? etc etc...normally can be answered relatively easily. It's generally an easy mod to do and once you know the basics you'll be able to do it no problem.

    For these few posts I'll concentrate on two questions and show a solution for both. With a few word swaps I'm sure these two examples can be broadened to cover quite a number of other questions/scenarios.

    1.) I want to install driving/fog lights and have them come on when my high beams are on. How do I wire this up?
    2.) I want to power my new spot lights. How do I wire them to a switch on my dash?

    Before we get answering the questions directly, let's look at a few of the components you'll need.

    ---------- Post added at 06:37 PM ---------- Previous post was at 06:10 PM ----------


    Think of a relay, essentially, as a switch. The switch works like any other - it turns things on and off. The only (major) difference is that to flick the switch you apply power to it or disconnect power to it.


    Well, relays can handle a lot more amperage than your usual automotive/12v switches. If you were to wire a regular switch directly to power your 100W spot lights you would melt your switch. Period. There would just be too much power going through the switch and most are not able to handle full amperage that is drawn from accessories.

    Relays, then, are used as a secondary switch that CAN handle all the amperage. All your dashboard switch is powering up, then, is the relay, which uses hardly any amperage - there's no load there to drawn any amps.

    Relays come in a few different configurations but the most common is the five terminal.

    The above picture is a four terminal.

    Check the packaging/configuration before buying. All look basically identical.

    For most applications what you are looking for is a relay that is Normally Open. This means that power needs to be supplied to the relay in order to toggle it on.

    Confusing, right? Let's have a look at it visually, maybe it will make a little more sense.

    In the above image is a typical five terminal relay. I dare say this is the one you will find 9 times out of 10.

    Numbers 85 and 86 are the coil power. These are the two terminals that need to be powered in order for the 'switch' to go on.

    85 is simply wired to ground, aka your vehicle's body/frame/engine.
    86 is wired to a source of power. This is the one you'd wire to the switch on your dash.

    Both of these will be relatively low in power draw. They do not require a large diameter wire.

    30 is the terminal that needs to receive high power rated wire gauge. This terminal will be supplying power, in the end, to the accessory you are hooking up. We'll look at amperage draw later on in the thread.

    87 and 87A are what will be wired to your accessory. In this five terminal relay 87A is Normally ON, and 87 is Normally OFF. As it sits, with nothing powering the 'trigger' power WILL go from terminal 30 to 87A. At rest this relay feeds power straight through. Powering this relay (switching it 'on', triggering it, etc) will CUT the power to 87A.

    Terminal 87 will NOT be receiving power until the relay is triggered. At that time power will flow from 30 to 87, powering your accessory.

    I hope this makes a little sense!

    Just remember...a relay is a switch that is powered on by electicity.

    More on purchasing a relay later in this thread!

    ---------- Post added at 06:47 PM ---------- Previous post was at 06:37 PM ----------


    Various switches can be purchased, and the sky is the limit on how they operate!

    Most people think of switches as ON/OFF but look closely when you buy!

    On the top in the picture you have regular automotive toggle switches.

    On the left you have an ON/OFF/ON switch. Meaning you can choose to power either one accessory or another, but not both at the same time. The toggle is in the middle when OFF then flipped in one direction of the other, depending on the accessory you want powered.

    To the right of that you have a lighted rocker switch. This is your simple ON/OFF switch but when it is power on it lights up to let you know.

    Both others, to the right of that, are mini switches - one toggle, the other rocker type. Both of these, with their three terminals, show that they can be wired either normally ON or normally OFF. Complicated but that means with one switch you can have an accessory powered and one the flick of the switch power diverts from one accessory to the other. Neither of the two accessories can be OFF at the same time.

    More on buying switches later in this thread!

    ---------- Post added at 07:02 PM ---------- Previous post was at 06:47 PM ----------


    Fuses are a big deal.

    Just know that., right?

    Fuses should be used in any circuit that is powered(+). If you have power from the positive(+) side of your battery it should have a fuse. WHEN (not if..but when) you get a short at some point, or if an accessory fails you will keep from having a major problem on your the very least melted wires and burned out worst a fire!

    Depending on how intricate your mod/installation is, you can either have simple single fuse holders:

    .....or a full buss with 4, 6, 10 fuses:

    Single fuse holders are easy to use, easy to wire in your circuit and are well protected.

    Most busses are NOT well protected but can eliminate a lot of wiring. Busses have one single power(+) wire going in and the different fused circuits going out. Soooo..with one power wire from the battery you can power, and have a fuse in line with, 6 circuits with the above buss.

    For any project you do on your vehicle, a spade-type fuse should be used.

    Lastly.....any circuit that is receiving power(+) SHOULD BE FUSED ON THE BATTERY SIDE, AND AS CLOSE TO IT AS YOU CAN. The longer you have a length of wire, the more chance you have of getting a short. TECHNICALLY there is no difference how far away from the battery the fuse is, as long as it's on the power(+) wire somewhere. But I always advise 'the closer the better'. If anything goes wrong, a fuse close to the battery will blow and ensure your entire circuit loses power, no matter where it is, no matter what it's touching, no matter what you have in your circuit.

    Make sense? No? Good...moving along then!

    ---------- Post added at 07:06 PM ---------- Previous post was at 07:02 PM ----------


    I don't know why but it seems a lot of people forget about the usefulness of plugs!

    They're cheap, they're very handy, and they're easy to install!

    Next time you're at a salvage/junk yard, go through a few cars and pull plugs! Get some two wire ones, three wires, 10 wires, one wire...whatever you can get! They'll come in handy at some point, I promise you. If've spent an extra few bucks on something to fill up your parts bin in your garage!

    When it's time to clean your fog lights you'll be happy to have used plugs for them so you can take them off without cutting any wires. If you need to quickly cut the circuit to any accessory, pull the plug!

    ---------- Post added at 07:21 PM ---------- Previous post was at 07:06 PM ----------

    Buying any of the above

    I just want to give you an idea of what to calculate before you go buying this stuff. How much amps are you going to draw? What size fuse?

    There's a simple equation you need to keep in mind, and this will serve you well in the future and will help you pick the right pieces to the puzzle:

    Watts \ Volts = Amp

    Keeping in mind that automotive electrical systems are generally in the range of 14V

    With lights that are 55w (EACH!!!), the example would be:

    110 \ 14 = 7.9

    So you're looking at a fuse one size up from 7.9A

    If you have lights that are 100w each:

    200 \ 14 = 15.7

    So a fuse greater than 15.7A

    The same goes for your relay. You have to make sure your relay can handle the power that will be going through it when your accessory is powered. Look on the packaging or look on the relay itself. It SHOULD tell you what the amp rating is on it, as on most electrical components:

    This relay should be fine to power your two 100W lights.
    Last edited by Springthing; 08-08-2010 at 07:56 PM.

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